Medical implant materials, in particular orthopedic implant materials, seek to combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys (CoCr), titanium alloys and more recently zirconium alloys which have proven to be the most suitable materials for the fabrication of load-bearing and non-load bearing prostheses.
To this end, oxidized zirconium orthopedic implants have been shown to significantly reduce polyethylene wear of the oxidized zirconium orthopedic implants articulating against a polyethylene surface. The use of oxide surfaces such as oxidized zirconium in orthopedic applications was first demonstrated by Davidson in U.S. Pat. No. 5,037,438. Previous attempts have been made to produce oxidized zirconium layers on zirconium alloy parts for the purpose of increasing their abrasion resistance. One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on Zircaloy 2 and Zircaloy 4. However, this procedure results in significant dimensional changes, and the oxide film produced does not exhibit especially high abrasion resistance.
U.S. Pat. No. 2,987,352 to Watson discloses a method of producing a blue-black oxide layer on zirconium alloy parts for the purpose of increasing their abrasion resistance. The blue-black color is the appearance of the zirconium oxide formed on the surface. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 3,615,885 produce a zirconium oxide layer on zirconium alloy by means of air oxidation. U.S. Pat. No. 3,615,885 continues the air oxidation long enough to produce a beige layer of greater thickness than the blue-black layer of U.S. Pat. No. 2,987,352. The beige appearance was sighted to be due to the fine micro-cracks on the surface of the oxide. The presence of micro-cracks may lead to spalling or removal of surface oxide particulates thus may not be applicable to many components where there are two work faces in the close proximity.
The blue-black layers have a thickness which is less than that of the beige layer although the hardness of the blue-black layer is similar to that of the beige layer. This blue-black oxide layer lends itself better to surfaces such as prosthetic devices. Although the blue-black layer is more abrasion resistant than the beige layer it is a relatively thin layer.
As discussed above, U.S. Pat. No. 5,037,438 to Davidson discloses a method of producing zirconium alloy prostheses with a oxidized zirconium surface. U.S. Pat. No. 5,180,394 to Davidson discloses orthopedic implants with blue-black zirconium oxide or zirconium nitride surfaces. U.S. Pat. No. 2,987,352 to Watson discloses a method of producing zirconium bearings with an oxidized zirconium surface. The oxide layer produced is not always uniform in thickness and the non-uniformity reduces the integrity of the bonding between the zirconium alloy and the oxide layer and the integrity of the bonding within the oxide layer. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 5,037,438 are incorporated by reference as though fully set forth herein.
In U.S. Pat. Nos. 6,447,550 and 6,585,772 and U.S. Patent Publication No. 2006/0058888, Hunter, et al. describes methods for obtaining an oxidized zirconium layer of uniform thickness. Hunter teaches that such is obtained by applying pre-oxidation treatment techniques and by manipulation of substrate microstructure. The use of uniform thickness oxide layer results in increased resistance to corrosion by the action of the body fluids as well as other benefits and is biocompatible and stable over the lifetime of the recipient. U.S. Pat. Nos. 6,447,550 and 6,585,772 and U.S. Patent Publication No. 2006/0058888 are incorporated by reference as though fully set forth herein.
Zirconium alloys are typically soft. The hardness of such alloys can range from 1.5 to 3 GPa. Since these alloys are soft, they can be easily abraded with a harder material. As described in the prior art, the abrasion resistance of zirconium alloys can be improved by oxidizing or nitriding these alloys. The significant reduction in wear of polyethylene against oxidized zirconium surfaces is attributed to the harder ceramic nature of the oxide. The hardness of the zirconium oxide surface is approximately 12 GPa. The oxidized zirconium implant typically has a 5 to 6 micron thick ceramic surface (zirconium oxide) that is formed by a thermally driven diffusion process in air. Below the zirconium oxide is a hard, oxygen-rich diffusion layer of approximately 1.5 to 2 microns. Below the diffusion zone is the softer zirconium alloy matrix. FIG. 1 shows a schematic cross-sectional view of such and oxidized zirconium structure taught by Davidson and Hunter (herein referred to “Davidson-type” oxidized zirconium) and FIG. 2 shows the hardness profile of the Davidson-type oxidized zirconium (M. Long, L. Reister and G. Hunter, Proc. 24th Annual Meeting of the Society For Biomaterials, Apr. 22-26, 1998, San Diego, Calif., USA).
Oxidized zirconium has been a great advancement over the conventional cobalt chromium and stainless steel alloys. There is still room for improvement. The totality of hardened zones (oxide plus diffusion hardened alloy) render the implant resistant to microscopic abrasion (for example, from third bodies such as bone cement, bone chips, metal debris, etc.) and slightly less resistant to macroscopic impact (surgical instrumentation and from dislocation/subluxation contact with metallic acetabular shells). The smaller hardening depth of these implants renders them less than optimal for hard-on-hard applications although such use has been suggested by Hunter and Mishra (U.S. Pat. No. 6,726,725). Hunter '725 teaches that the oxide thickness can be increased up to 20 microns for such applications. But Davidson-type oxide compositions having such thicknesses, although highly wear-resistant, can have significant number of oxide layer defects when the oxide thickness is increased to 20 microns. Such defects can lead to localized spalling of the oxide. Also, in the Davidson-type composition below the oxide, there is a relatively small diffusion hardened zone. Thus, while the Davidson-type compositions exhibited superior wear resistance compared to many conventional materials, there is room for improvement.
In a hard-on-hard application such as in a hip joint, the material articulates against itself or another hardened or non-hardened metal instead of polyethylene. The wear rates in such types of implants could be as high as 1 micron per year. With the totality of the hardened zone (oxide and diffusion zone) having a thickness of less than 7 microns (approximately 5 micron oxide and 2 micron diffusion hardened zone below the oxide), Davidson-type oxidized zirconium implants, although representing the state-of-the-art when originally introduced and still quite useful, have room for improvement in such applications.
Currently, there are two primary types of hard-on-hard hip implants that are available commercially, namely metal-on-metal and ceramic-on-ceramic. The current standard material of metal-on-metal implants is high carbon Co—Cr alloy. The major concern with the metal-on-metal implant is the metal ion release from the joint and its unknown effects on the physiology of the human body. The advantage of metal-on-metal implants is that they can be used in larger sizes. The larger size of the implant allows greater range of motion and stability of the implant. The metal-on-metal implants have also been shown to be useful for resurfacing type of application where conservation of bone is desired. In such larger joints, the conventional or cross-linked polyethylene is not preferred and metal-on-metal may be the only choice available. The larger size requires polyethylene liner to be thinner. A thinner liner may not be mechanically strong, may creep more or may lead to increased wear and osteolysis and eventually failure of the implant. The other commonly used hard-on-hard implant material is ceramic-on-ceramic. The current standard material of ceramic-on-ceramic implants is alumina. The surface hardness of the alumina is approximately 20 to 30 GPa. Metal ion release is typically not a concern for these implants. But due to limited toughness and the brittle nature of ceramics, it is difficult to make these implants in larger sizes. The ceramic components have finite probability of fracture thus leading to a potential joint failure and complications associated with the fracture of a joint.
One of the ways to improve the surface hardness of oxidized zirconium is to form a zirconium nitride instead of zirconium oxide. Kemp (U.S. Pat. No. 5,399,207) describes a method to make oxidized or nitrided zirconium compositions using a fluidized bed furnace. Kemp states that the nitridation can be carried out from 1300° F. (700° C.) to 1600° F. (870° C.). Kemp teaches use of pure nitrogen instead of air or oxygen to achieve the nitridation of the surfaces. U.S. Pat. No. 5,180,394 to Davidson discloses orthopedic implants with blue-black zirconium oxide or zirconium nitride surfaces. Note that zirconium nitride typically appears yellowish-golden and thus can be distinguished from blue-black zirconium oxide. Davidson teaches that the nitride layer to be formed at 800° C. in about one hour in nitrogen atmosphere. Use of such high temperature can lead to micro-structural changes such as grain growth. These changes in-turn may affect the mechanical properties of the substrate. Higher temperature process can also dimensionally distort the components being manufactured. It should be noted that the zirconium nitride may not adhere as well as zirconium oxide does to the zirconium alloy substrate. It should also be noted that in all the prior art, the methods employed make zirconium oxide or make zirconium nitride.
The art of increasing depth of hardening in titanium alloys has been described previously. It involves, basically one of forming an oxide on the surface of the article by treatment in an oxygen-rich environment, followed by heat treating the article in an oxygen-deficient inert environment. One of the approaches provided by Takamura (Trans JIM, vol. 3, 1962, p. 10) has been to oxidize a titanium sample followed by treating it in an inert gas such as argon gas. This apparently allows oxygen to diffuse in the substrate and form a thick diffusion zone. Presence of oxygen in the diffusion zone leads to hardening. Another approach suggested by Dong et al. (U.S. Pat. No. 6,833,197) is to use vacuum or an inert gas to achieve an oxygen-deficient environment, thereby achieving the diffusion-hardening after oxidation. The preferred temperature specified by both Takamura and Dong et al for oxidation is 850° C. and that for diffusion hardening (vacuum treatment) is 850° C. Use of such high temperature can lead to micro-structural changes such as grain growth. These changes in-turn may affect the mechanical properties of the substrate. Higher temperature process can also dimensionally distort the components being manufactured. Dong et al suggest this methodology for titanium and zirconium and titanium/zirconium alloys. One of the problems with these methods, particularly for zirconium alloys, is that the oxidation and diffusion hardening temperatures are significantly high and can lead to thick and cracked (defective) oxide as well as cracks in the substrates after diffusion hardening. Dong demonstrates its method using titanium alloys; no examples for zirconium/niobium-based or titanium/zirconium/niobium-based alloys have been shown. Further, both Dong and Takamura require complete dissolution of the oxide in the substrate in an inert atmosphere such as argon or in vacuum. Dong also teaches a sigmoid shaped hardness profile of the diffusion hardened metallic zone as a result of this complete dissolution of the oxide. The sigmoid shaped diffusion hardened zone profile requires complete dissolution of the oxide in the substrate. Another approach is suggested by Pawar et al. (U.S. Patent Application No. 20070137734). Pawar et al. teach a method to obtain depth of hardening and a defect-free ceramic surface by carefully controlling the temperature and time of oxidation and of the vacuum diffusion treatment. Pawar et al. also teach the method to form an error-function type layered hardness profile of the diffusion hardened zone. However, the method teaches formation of only one type of defect-free ceramic layer such as oxide or nitride. U.S. Pat. No. 6,833,197 and U.S. Patent Publication No. 20070137734 are incorporated by reference as though fully set forth herein.
Previously shown methods either increase the surface hardness at the expense of lowering the adherence of the surface layer to the substrate and limiting the depth of hardening, or, conversely, increase the depth of hardening by heat treating in an inert gas atmosphere at the expense of creating surface deformations and adversely affecting surface hardness. In some methods defect-free ceramic layer is obtained with increased depth of hardening. In all these methods, the surface has only one ceramic layer. The inventors of the present invention have discovered that instead of using vacuum or inter atmosphere, a reactive gas can be employed. This reactive gas in-turn will transform the surface layer to the different type of ceramics such as nitrides or oxides or oxynitrides. This produces a layered ceramic structure of a composition which has not been shown in the prior art. This layered ceramic structure thus can be tailored to take advantage of the ceramic surface formed. For example, the first layer on the surface could be hard zirconium nitride which is a highly reflective golden appearance surface. The layer underneath this nitride could be blue-black zirconium oxide. Several such combinations of layers can be made to achieve the surface hardness and the specific characteristics of those ceramic layers.